Designing mechanically reinforced filler network for thin and robust composite polymer electrolyte

Developing novel solid electrolytes with high performance is of great significance for the practical application of lithium metal batteries. Among all the developed solid electrolytes, composite polymer electrolytes (CPEs) have been deemed one of the most viable candidates because of their comprehensive performance. Nevertheless, the random distribution of inorganic filler nanoparticles may cause discontinuities in ion transport and low mechanical strength. Therefore, the introduction of a filler network with fast ion conduction is an effective strategy to provide continuous ion transport and mechanical support. The mechanically reinforced filler network enhances the mechanical strength of the CPE, providing opportunities to reduce the thickness of CPE. In this review, the progress of mechanically reinforced filler structures in CPE is summarized, along with the introduction of mechanically reinforced filler networks with random and ordered structures and electrode‐integrated CPE with mechanically reinforced filler networks. Finally, challenges and possible future research directions for mechanically reinforced filler network CPE are presented.


| INTRODUCTION
][4] However, the utilization of Li metal anodes brings considerable safety issues due to its continuous side reactions with liquid electrolytes and the uncontrollable dendrite formation.[7][8][9][10] Solid-state electrolytes (SSEs) are normally divided into two categories: inorganic solid electrolytes (ISEs) and solid polymer electrolytes (SPEs).Among them, ISE is advantageous in terms of high ionic conductivity, mechanical strength, and thermal stability, but is limited by the poor interface compatibility with electrodes. 11,123][24][25][26][27][28] Fillers are reported to promote ionic conductivity by lowering polymer crystallinity or/and creating additional ionic pathways.0][31][32] However, fillers in powder form tend to agglomerate and precipitate in the polymer matrix, which disturbs ion transport and causes a limited improvement in ionic conductivity.][35][36][37][38][39][40] Besides ionic conductivity, mechanical property is another key characteristic of SSE that determines battery safety.Lower crystallinity offered by inorganic fillers would result in not only rapid ionic conductivity but also mechanical strength reduction of the polymer matrix, which in turn affects the safety of the battery. 41To address this dilemma, the design and construction of a versatile mechanically reinforced filler network is an effective strategy. 42,43The mechanically reinforced filler network provides mechanical support for the soft polymer phase and improves the mechanical properties of the electrolyte membrane. 44In addition, the filler network combines with the polymer through a monolithic structure, which avoids filler uneven distribution in the polymer matrix, and provides a continuous and undisturbed channel for ion conduction. 42,45,46Therefore, the mechanically reinforced filler network infused with the flexible polymer creates robust SSE membranes with high ionic conductivity.The preferable mechanical properties of such SSE allow the possibility of membrane thinning, which greatly enhances the energy density of all-solid-state batteries.
In this review, we summarize the research progress of CPEs with mechanically reinforced filler networks and present the challenges and prospects of future research directions.First, we describe the mechanically reinforced filler network with random structure and then divide them into passive and active filler networks.Commercially available porous separators and biomass materials are also introduced as low-cost supporting scaffolds for soft polymer electrolytes or brittle inorganic electrolytes.Then, the mechanically reinforced filler network with ordered structure is summarized to provide an enhancement effect in ionic conductivity, mechanical strength, or multiple properties of the resulting CPEs.The ordered structures are categorized into horizontal and vertical ordered networks.The function of electrode-integrated CPEs with a reinforced filler network was then discussed (Figure 1).To further identify the role of these CPEs in the performance of solid-state batteries, Table 1 summarizes state-of-the-art CPEs, along with significant structural parameters and corresponding battery performance.Finally, we provide our insights into the current challenges and future prospects of mechanically reinforced filler networks for solid-state lithium batteries.

| MECHANICALLY REINFORCED FILLER NETWORK WITH RANDOM STRUCTURE
The introduction of a mechanically reinforced filler network is an effective strategy for fabricating highperformance SSE, 64 and various robust membranes have been utilized as physical supports for CPEs.Mechanically reinforced filler network with random structure forms interconnected filler/polymer interphases and effectively eliminates the degradation of ionic conductivity due to filler agglomeration.The intertwined filler network can also reduce local strain energy by redistributing the localized stress concentration, leading to enhanced mechanical resilience. 16,65Furthermore, the puncture strength of the CPE serves as a criterion for assessing dendrite growth resistance and can be determined through stress-strain testing of the electrolyte membrane.The   66 The improved ionic conductivity and mechanical properties of the resulting CPEs inhibit the growth of lithium dendrites and promote long-term cycling stability. 671 | Passive filler network with random structure 0][71] The isolated inorganic particles inside the CPE cannot form a continuous Li + transfer network, and the effect of enhancing Li + conductivity will be greatly reduced.Li et al. investigated the impact of ceramic-filled nanoparticle content in polymer electrolytes on ionic transport.Randomly distributed nanoparticles failed to establish lithium grain boundaries for efficient transport, leading to a limited enhancement in ionic conductivity. 703][74] However, one-dimensional (1D) nanowires with higher specific surface areas are more easily to agglomerate in the polymer matrix compared to the zero-dimensional (0D) nanoparticles, and the aggregated fillers disturb ion transport and degrade ionic conductivity.6][77] In this sense, a filler network made of randomly connected nanowires becomes a promising strategy to simultaneously enhance both ionic conductivity and mechanical properties of the CPEs.
The electrostatic spinning method has been extensively investigated to fabricate filler networks composed of connected 1D nanowires. 78,79In such a method, it is convenient to regulate the network structure and composition by tuning the ratio of polymer matrix to filler phase in the preparation of precursor solution, Liu et al. produced flexible and robust SiO 2 nanofiber (NF) films and constructed a concrete CPE by incorporating poly(ethylene oxide)-LiN(SO 2 CF 3 ) 2 (PEO-LiTFSI) into the porous inorganic frame (Figure 2A). 80The flexible SiO 2 NF films were prepared by sol-gel electrospinning and then calcined at a low temperature.The CPE made of SiO 2 NF film and PEO/LiTFSI polymer electrolyte was fabricated by the liquid injection method by directly immersing the SiO 2 film in PEO-LiTFSI solution followed by volatilizing the solvent.The CPE has a high room temperature (RT) conductivity of 1.3 × 10 −4 S cm −1 (Figure 2B), which is an order of magnitude higher than that of the pure PEO-LiTFSI electrolyte.In addition, the mechanical strength of CPE is about double that of the PEO-LiTFSI films (Figure 2C).The high mechanical strength allows the CPE membrane to be reduced to 28 μm thick, which is beneficial for reducing the internal resistance and thus accelerating the ion conduction.The charge transfer resistance of the CPE is half that of the PEO polymer electrolyte and the Li|CPE|Li symmetric cell stably cycled for more than 500 h of cycling at a current density of 1 mA cm −2 at 60°C.4][85] Meanwhile, its nanoscale pores can selectively confine larger anions for fast Li + transport.Therefore, the interconnected MOF network is expected to further enhance the Li + conductivity.Li et al. developed a CPE based on a 3D interconnected MOF network (Figure 2D). 81The interconnected MOF network was constructed by electrostatic spinning of MOF nanoparticles embedded in PAN and then backfilled with liquid injection to form CPE (~100 μm thick).In addition, the assembled symmetric Abbreviations: AAO, anodic alumina; CPE, composite polymer electrolyte; GFC, glass fiber cloth; IL, ionic liquid; LAGP, Li  cell is stable to cycle for more than 700 h at a current density of 0.3 mA cm −2 at 60°C.By combining MOF with ionic liquid (IL), MOF can be turned into a fast ionic conductor to realize the rapid transport of Li + .Du et al. formed a continuous linear Li + channel along the direction of the polyimide fiber through the directional self-assembly of MOF and polyimide fiber and the combination of ILs (Figure 2E). 82MOFs are in situ assembled to produce a layered self-assembled MOF network by etching the surface of polyacrylic fibers.By encapsulating the IL containing lithium in the hole of the MOF, the original MOF network is changed into a 3D ionic conductor.Ultimately, a Li + conductive MOF network was penetrated with the PVDF-LiTFSI polymer electrolyte to create CPE.CPE has a low activation energy of 0.10 eV and a high ionic conductivity of 6.17 × 10 −4 S cm −1 at RT (Figure 2F).The tensile resistance of PVDF-based polymer electrolytes ~4.98 MPa is further enhanced by the addition of layered selfassembled MOF networks ~22.02MPa (Figure 2G).

| Active filler network with random structure
Active fillers not only reduce the crystallinity of polymers but also provide an additional ion-conducting pathway. 86herefore, it is generally believed that the active filler can improve the ionic conductivity of CPE more effectively. 87owever, the homogeneous distribution of the active fillers in the polymer matrix is also challenging, and the mechanical strength of the resulting CPE is not satisfactory. 88,89][92] Electrospinning technology combined with pyrolysis is widely used to prepare the active filler network. 50For example, Liu et al. fabricated a CPE with high significant mechanical strength and low contact resistance by infiltrating the Li 0.33 La 0.557 TiO 3 (LLTO) NF network with PEO-LiTFSI solution (Figure 3A). 51The precursor solution composed of polyvinylpyrrolidone polymer and precursor salt is electrospun to obtain a flexible film, followed by calcination to form an LLTO NF network.The assembled symmetrical battery has a stable cycle of 420 h at the current density of 0.3 mA cm −2 at 60°C.To further enhance the mechanical properties, Li et al. mixed LATP nanoparticles with PAN and fabricated LATP/PAN composite fiber network via electrospinning, followed by a liquid injection method to obtain the CPE (Figure 3B). 93ompared with polymer PEO 8 -LiTFSI electrolyte, 2LATP/ PAN-[PEO 8 -LiTFSI] has higher mechanical properties (tensile strength of 10.72 MPa) (Figure 3C).Moreover, the conductivity of CPE is greatly increased and four times greater than that of PEO 8 -LiTFSI electrolyte (Figure 3D), and the assembled symmetrical battery stably cycled for more than 400 h under the current density of 0.3 mA cm −2 at 60°C.Bacterial cellulose (BC) with highly networked nanofibrils and water adsorption capacity is also used to absorb the precursor salt solution and produce an interconnected ceramic network after calcination.For instance, a cubic LLZO (c-LLZO) network can be synthesized by submerging BC film in the c-LLZO precursor solution followed by calcination (Figure 3E). 52he effective synthesis of CPE was accomplished by combining c-LLZO film with PEO-LiTFSI polymer.The high mechanical strength reduces the thickness of CPE to 70-100 µm, which is much thinner than the conventional c-LLZO bulk electrolyte >500 µm).
Combining filler networks with in situ polymerization can effectively improve the interfacial stability of filler/ polymer composites, which ensures the formation of fast ion conductive tunnel. 95Zheng et al. obtained the LLTO NF framework by electrospinning and pyrolysis and then prepared CPE by in situ cationic polymerization of 1,3dioxolane to obtain poly (1,3-dioxolane) in the selfsupporting LLTO NF framework (Figure 3F). 94The in situ formed CPE is attached to the mechanical supporting frame, and its thickness is greatly reduced (~10 μm) (Figure 3G).This structure enables the CPE-based LiFePO 4 (LFP) full battery to cycle stably for 350 cycles at a current density of 0.5 C at RT.In addition, Ren et al. obtained Li 6.4 La 3 Zr 1.4 -Ta 0.6 O 12 (LLZTO) composite PAN network structure through electrospinning and pyrolysis.The PEP was injected into the film, and SSE was formed through thermal polymerization (Figure 3H). 33The polymer electrolyte is firmly locked in the LLZTO network structure, further improving the mechanical strength of CPE and reducing the film to 80 μm thick.The fiber network wrapped LLZTO also facilitates the migration of Li + and immobilizes the anion through the Lewis acid-base effect, which promotes the rapid transport of Li + with high ionic conductivity and low activation energy (Figure 3I).Moreover, the assembled LFP full battery can cycle stably for more than 500 cycles at a current density of 0.5 C at 60°C.

| Commercial porous structure
][55][56] This kind of CPE is promising for commercialization due to its facile and scalable fabrication, low cost, and wide range of raw material sources.For instance, Wu et al. prepared a PEO-based CPE by infiltrating PEO/LiTFSI into the PE separator. 96The thickness of the CPE membrane is reduced to 7.5 μm with the help of the robust and flexible PE separator (Figure 4A).Compared with PEO/LiTFSI SSE, the mechanical properties of PEO-CPE electrolytes were significantly improved, and Young's modulus increased by more than three orders of magnitude (Figure 4B).The symmetrical battery is stably cycled for more than 400 h at a current density of 0.15 mA cm −2 at  93 Copyright 2018, American Chemical Society.(E) Schematic demonstrating the procedure for the synthesis of the hybrid electrolyte with bacterial cellulose as an inexpensive, scalable, and efficient template. 52Copyright 2018, Wiley-VCH.(F) Schematic illustrating the preparation of the in situ composite polymer electrolyte (CPE).(G) Cross-sectional views of the cycled Li/LiFePO 4 batteries. 94Copyright 2022, The Royal Society of Chemistry.(H) Schematic diagram of lithium dendrite growth mechanism with CPE.(I) Temperature-dependent ionic conductivity of CPE. 33Copyright 2022, Elsevier.60°C.To further improve the interfacial affinity between the CPE and electrodes, Wang et al. used the phase inversion method to attach a thin layer of porous polymethyl methacrylate polystyrene (PMMA PS) on both sides of the PE separator (Figure 4C). 97The interface layer of PMMA PS is tightly attached to both sides of PE, effectively improving the interfacial compatibility between the electrolyte and electrode, thus enabling high-capacity F I G U R E 4 (A) Cross-sectional scanning electron microscope (SEM) images of the separator after poly(ethylene oxide)-LiN(SO 2 CF 3 ) 2 (PEO/ LiTFSI) infiltration.(B) Stress-strain curves of different films. 96Copyright 2019, Wiley-VCH.(C) Schematic diagram of preparation process for the composite polymer electrolyte (CPE). 97Copyright 2021, Wiley-VCH.(D) The schematic illustration of the lithium symmetric batteries and solidstate batteries before and after cycling.(E) Ionic conductivity of CPE at different temperatures. 98Copyright 2021, Elsevier.(F) Photographs of flat and bent PEO-SN 25 -LiTFSI 10 -glass fiber (GF) electrolyte.(G) Stress-strain curves of PEO-SN 25 -LiTFSI 10 -GF. 99Copyright 2021, American Chemical Society.(H) Schematics of the CNF/PEO solid polymer electrolyte. 100Copyright 2020, Elsevier.(I) Cross-sectional SEM images of bacterial cellulose-CPE (BC-CPE).(J) Ionic conductivities of BC-CPE.(K) Tensile stress-strain curve of BC-CPE.The inset shows the flexibility of BC-CPE under bending. 101Copyright 2022, IOP Science.CNF, cellulose nanofibrils; IL, ionic liquid; PL, polymer electrolyte; PPL, polyethylene/polymer electrolyte.retention at higher current densities (0.1 mA h g −1 , 7 C/105 mA h g −1 ).The symmetrical battery is stably cycled for more than 1500 h at a current density of 0.1 mA cm −2 at 60°C.Glass fibers with large pore volume and strong permeability to electrolytes are also reported to support soft polymer electrolytes.Zhang et al. prepared a 3D fiber network-reinforced CPE by impregnating PEO, LiTFSI, and ILs in GFC (Figure 4D). 98The presence of GFC effectively inhibits the crystallization of PEO and improves its ionic conductivity (Figure 4E).At 30°C, the ionic conductivity was 1.1 × 10 −5 S cm −1 , and the symmetrical battery remained stable for more than 2000 h (60°C and 0.2 mA cm −2 ).A similar structure was fabricated by Wang et al. by combining PEO, SN, and GFC (Figure 4F,G), which mechanical properties significantly higher than discontinuous and less ductile aerogel fillers. 99][104][105][106][107] For example, cellulose nanofibrils containing negatively charged surface carboxylic acid groups were used to support polymer electrolytes (Figure 4H). 100 The surface functional groups interact with Li + through electrostatic interactions and accelerate the ion transport.Due to the improved mechanical properties and ionic conductivity, a sustained cycle at 60°C and a current density of 0.5 mA cm −2 in the symmetrical battery.Similarly, the BC framework was used as a support matrix to prepare BC-based CPE. 101After a solvent exchange, BC hydrogel was dried to produce BC membrane, and the BC film is immersed in the precursor solution containing poly (ethylene glycol) diacrylate (PEGDA), SN, and LiTFSI.The CPE of 30 μm thick is produced by photocuring using ultraviolet lamps (Figure 4I).BC-CPE has an RT ionic conductivity of 1.3 × 10 −4 S cm −1 (Figure 4J) and a tensile resistance of 7.25 MPa (Figure 4K).It is worth noting that the polymer electrolyte in such CPEs often experiences issues like detachment, decomposition, and melting at elevated temperatures, which can compromise battery reliability under high-temperature conditions.Consequently, researching polymer electrolytes with higher melting points can effectively enhance the operational temperature range of commercial porous filler CPEs.

| MECHANICALLY REINFORCED FILLER NETWORK WITH ORDERED STRUCTURE
Mechanically enhanced random structured filling networks can enhance the ionic conductivity of CPE without affecting the mechanical strength.9][110] Therefore, mechanically enhanced filling networks with ordered structures can further achieve uniformity of ion transport in the electrolyte and accelerate ion transport. 111,112In addition, the ordered filler structure can dissipate the mechanical impact energy through the interface and plastic deformation, thus enhancing the toughening effect.This facilitates further thickness reduction and inhibits dendrite growth to extend cell cycle life. 25,67,113

| Horizontally ordered structures
8][119] In addition, this horizontal structure allows the CPE to have robust mechanical integrity at a low thickness and can be safely stacked onto the electrodes for the fabrication of standard solid-state batteries. 108,120he highly arranged horizontal structure provides an enhanced toughening effect and good flexibility, effectively resisting dendritic growth and stress-strain during cycling.For example, Li et al. produced a nacre-like CPE composed of ceramic layers and polymer binders (Figure 5A). 57The thickness of each ceramic layer is about 20 μm.This structure with ceramic electrolyte and elastic polymer matrix is expected to achieve the coexistence of flexibility and high mechanical strength of CPE (Figure 5B,C).Besides, a much larger ultimate flexural modulus can be obtained for the nacre-like solid electrolyte compared to pure polymer electrolytes.In addition, the failure strain of CPE is about 1.1% (Figure 5D), which is about an order of magnitude higher than the failure strain of pure phase films (0.13%).The RT ionic conductivity of this CPE is 1.25 × 10 −4 S cm −1 , and the capacity retention rate of the LFP full battery at 60°C and 0.5 C is 92% after 300 cycles.
In addition, electrospinning provides the possibility for large-scale production of horizontally oriented structures, and spinning can make the distribution of filler structures more uniform, thereby providing horizontally anisotropic Li + conduction.In addition, nanowires parallel to the electrode can provide more orderly interface contact, thereby improving ion conductivity.Guo et al. prepared CPE films of interwoven LLZO/PEO LiTFSI microfibers using the coaxial electrospinning method (Figure 5E). 58CPE has high tensile strength, which can be attributed to the mechanical effect of uniformly dispersed LLZO and PEO-LiTFSI interweaved microfibers (Figure 5F).Ion conductivity of this CPE reaches 1.5 × 10 −4 S cm −1 at 35°C, approximately 60 times that of PEO-LiTFSI.A similar structure was created by electrospinning horizontally arranged LLZO NFs followed by incorporating PVDF/lithium perchlorate polymer electrolytes (Figure 5G). 121The thickness of solid electrolytes significantly impacts the energy density of batteries.The thickness of the composite electrolyte film can be controlled at 70 μm (Figure 5H).The assembled symmetrical battery undergoes a temperature cycling of 700 h at a current density of 0.5 mA cm −2 at RT.In summary, the interaction between the well-arranged inorganic skeleton structure and the polymer can provide a long and smooth conduction pathway for Li + , enabling CPE to have high ion conductivity and strong mechanical properties.This provides a promising strategy for producing better SSEs and achieving the commercialization of solid-state batteries.

| Vertically ordered structures
Compared to the horizontally ordered structure, vertically ordered filler networks can provide shorter ion transfer pathways due to the direct ordered ion transport channels in between electrodes, which further enhances ion conductivity. 32he template method has been extensively utilized to produce CPE with a vertically ordered structure.The template is usually removed after the vertical array is formed, and a second phase (mostly polymer electrolyte) is introduced to bring in flexibility for the CPE. 122For instance, Dai et al. used wood as a sacrificial template to fabricate vertically aligned LLZO arrays with low tortuosity. 59LLZO precursor was infiltrated into the microchannels of wood followed by high-temperature annealing to form a vertically arranged garnet structure.The well-arranged structure of natural wood can be well inherited in the final LLZO network (Figure 6A,B).After incorporating PEO polymer electrolytes into the LLZO arrays, a CPE with a well-aligned ion transport pathway is obtained with an enhanced RT of ion conductivity of 2.2 × 10 −4 S cm −1 (Figure 6C).The assembled symmetrical battery has a stable over potential of 50 mV for 180 h at 0.1 mA cm −2 and RT.The ice template method is a widespread approach for fabricating SSEs with controlled architectures.Randomly dispersed nanoparticles can be converted into a vertically aligned structure along the icegrowing direction (Figure 6D). 32If only the bottom of the dispersion is cooled, the vertical temperature gradient is formed from bottom to top, and ice crystals gradually grow upward and push the ceramic to particles to form a vertically arranged structure.After the ice template is removed, CPE is obtained by injecting the polymer electrolyte solution into the inorganic arrays.The ionic conductivity of such CPE reaches 5.2 × 10 −5 S cm −1 (Figure 6E).The garnet skeleton with low tortuosity can provide vertical and continuous Li + conduction pathways, which considerably enhances the ionic conductivity of the resultant CPE.
On the other hand, some templates remained in the final CPE to reinforce the mechanical properties, as well as to optimize the ion transport interphases.For instance, Zhang et al. used AAO as a template to fabricate a CPE with vertically arranged and continuous ceramic/polymer interfaces by infiltrating polymer electrolytes into the nanochannels of AAO (Figure 6F). 60The combination of the rigid AAO and flexible PEO results in a thinner CPE membrane (60 μm) with enhanced dendrite suppression capability.The assembled symmetrical battery has a stable over potential of 40 mV for 400 h at 0.75 mA cm −2 and RT.Since Li + can be rapidly transported along the vertically arranged interface, the ion conductivity reaches 1.79 × 10 −4 S cm −1 at RT (Figure 6G).In addition, polymers with vertically aligned pores can also be used as a template and mechanical support to hold polymer electrolytes. 123The PI membrane possesses vertically aligned nanochannels and a thickness of 8.6 μm (Figure 6H).In addition, the PI membrane is nonflammable and mechanically robust (Figure 6I), and the Li-Li cells are stable over 1000 h without short circuits (0.1 mA cm −2 and RT).Because of the vertically aligned nanochannels, this polymer-polymer composite electrolyte delivers a highly enhanced ionic conductivity, and the Li + tends to transport along the vertical alignment direction, which explains the improved ionic conductivity of CPE in the aligned channels.

| INTEGRATED STRUCTURE SOLID-STATE BATTERY
Poor solid-solid contact between the SSE and electrodes has always been the bottleneck of solid-state battery performance.The inferior interface would disrupt ion transfer and reduce mechanical properties, resulting in capacity degradation.By constructing an all-solid-state battery with an integrated structure, the electrolyte can be effectively attached to the electrodes, thereby reducing interfacial resistance and improving the mechanical strength of the electrolyte.The mechanically supported SSE in the integrated structure can effectively enhance the solid-solid interfacial contact, reduce the internal resistance of the cell, and improve the efficiency of electron, ion, and charge transfer.
The integrated structured solid-state batteries are designed from both the cathode/anode side and allow for a significant increase in the mechanical strength of the battery.By infiltrating the polymer electrolyte into the gap of the electrodes grown directly on the collector, the electrolyte can be made dependent on the mechanical cosupport of the electrodes and the collector.In addition, electron conduction in the electrodes can be independently ensured by direct transport along the continuous 3D framework and direct transfer at the electrode material-collector interface.For instance, Liu et al. permeated the poly(vinylidene fluorideco-hexafluoropropylene) electrolyte into the spacer of the 3D TiO 2 electrode grown directly on the current collector (Figure 7A). 61The capacity remains 197.5 mAh g −1 with an excellent capacity retention of 86.8% after 800 cycles at 0.2 C.Meanwhile, Lin et al. fabricated ultrathin and robust fiber network-reinforced solid electrolytes on the cathode (Figure 7B). 62The fiber-reinforced PEO/garnet electrolyte well inherits the good mechanical properties of the robust PVDF fiber network (Figure 7C), and the electrolyte thickness on the cathode is only about 17 μm (Figure 7D).The capacity retention rate of the LFP full battery at 60°C and 0.5 C is 84.3% after 500 cycles.
Solid-state batteries have to be matched with lithium metal anode to have high energy density, but lithium metal cannot exist stably the air, and its assembly and transportation bring safety hazards.The integrated design of SSE and lithium metal anode can not only enhance the interface and air stability, which is beneficial to the battery cycle and safety.Zhang et al. fabricated the integrated structure of polyethylene glycol methacrylate-Li 1.5 Al 0.5 Ge 1.5 (PO 4 ) 3 -lithium by in situ copolymerization of organic-inorganic hybrid solid electrolyte on lithium foil. 63he SSE is composed of an 8.5-µm-thick with improved ionic conductivity of 2.37 × 10 −4 S cm −1 at RT, favoring high gravimetric and volumetric energy density (Figure 7E,F).The integration-reinforced cells retain the highest CE of 96.8% over 200 cycles.In comparison to the cathode side, the anode side of lithium is more active, which makes it difficult to stabilize contact with solvent-containing electrolytes, relying instead on the self-polymerization of polymer monomers to form integrated structures.Therefore, the development of simpler and cheaper polymer monomers is particularly important for industrial applications.

| SUMMARY AND PERSPECTIVE
This article reviews the research progress of CPEs with mechanically reinforced filler networks.According to the structure of the mechanically reinforced filler networks is divided into random and ordered structures CPE.In the random structure, the passive filler network, the active filler network, and the low-cost supporting scaffolds (commercially porous separators and biomass materials) are presented.In the ordered structure, the horizontally and vertically ordered filler network structures are presented.Finally, the enhancement of electrode-integrated CPEs with reinforced filler networks on the performance of all-solid-state batteries was discussed.Despite the rapid progress in the study of mechanically enhanced filler network CPE, significant challenges still exist and considerable efforts are required for a deeper understanding and further optimization of the filler network structure, including the following (Figure 8): i. Li + transference number to be improved.A low Li + transfer number leads to the creation of an ion concentration gradient, resulting in polarization and thus the formation of lithium dendrites.However, the enhancement of cation mobility is inevitably accompanied by the obstruction of anion migration, which will cause the ionic conductivity of CPE to decrease.Further development and modification of CPE are needed to balance the contradiction between high cation mobility and high ionic conductivity at the same time.ii.Development of ordered structured SSE with desirable structures.CPEs with ordered fillers are attractive in terms of the obvious enhancement effect on ionic conductivity.However, the polymer components tend to degrade at high temperatures, causing the CPE to break down.Enhancing the combination of filler and polymer through structural design is the primary focus of investigations to address this issue.In addition, it is necessary to control the ratio of filler to polymer in CPE and optimize the production method and process of the ordered structure to obtain CPE with a desirable ordered structure.puncture strength tests a more direct method to assess the CPE's ability to resist dendrite infiltration.v. Compatibility of CPE with high-voltage cathode.
High-voltage solid electrolytes are the key to building solid-state batteries with a long cycle life and high energy density.However, the currently available high-voltage solid electrolytes cannot meet the requirements of commercialization.In addition, the high-voltage cathode materials in a highly delithiated state exhibit strong oxidation activity, which accelerates the decomposition of SSEs.Due to the oxidative decomposition of the CPE, the interface between the high-voltage cathode and CPE can become a "bottleneck" for ion transport.Therefore, the modification of high-voltage solid electrolytes and improving the compatibility of CPE with high-voltage cathode is one of the key research priorities for the future.
iii.Advanced characterization techniques to reveal the deep mechanisms.The mechanically supported filler phase greatly improves the comprehensive performance of CPE, and a deeper understanding of its performance deserves attention.Exploring and analyzing the interfacial and physical/chemical interactions between the filler and the polymer matrix requires advanced techniques.In addition, as CPE is a complex multiphase system, reasonably combining different calculation methods and theories to realize the simulation of multiscale and multiphysics fields in CPE will become a growing trend in the future.iv.Compatibility of CPE with Li anode.The interface between the CPE and Li metal anode is a significant hurdle for the development of solid-state Li metal batteries.Unsuitable CPE/Li metal anode interface, which leads to the growth of Li dendrites and the repeated consumption of electrolytes.In addition, resistance to dendrite formation is a key indicator of CPE, especially in energy storage systems such as lithium-metal batteries.Dendrites in lithium-metal batteries can penetrate the separator, leading to internal short circuits or overheating.The robust mechanical properties of CPE can reduce the risk of dendrite penetration.Therefore, conducting F I G U R E 8 Perspectives for future research on composite polymer electrolyte (CPE).